Process for the production of melamine

ABSTRACT

A process for the production of melamine by pyrolysis of urea in a high-pressure reactor having a vertical central pipe is provided. The melamine flows upwards into the reactor from below, mixes in the lower part of the reactor with a urea melt, and optionally NH 3 , introduced into the reactor from below, and emerges from the central pipe in the upper part of the central pipe. Part of the melamine formed flows downward in the annular space between the central pipe and reactor wall, and the remainder is expelled for further work-up. The off-gases are removed at the top of the reactor. A reactor for carrying out the process is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.10/399,637, having a 371 filing date of Oct. 7, 2003, which is aNational Phase Patent Application of International Application NumberPCT/EP01/11890, filed on Oct. 15, 2001, which claims priority ofAustrian Patent Application Number A 1802/2000, filed Oct. 20, 2000.

FIELD OF THE INVENTION

The invention relates to a process and a device for the production ofmelamine by pyrolysis of urea.

BACKGROUND OF THE INVENTION

In the high-pressure processes for the production of melamine, urea isreacted to give melamine by means of an endothermic liquid-phasereaction. The liquid melamine, depending on the pressure and temperatureconditions in the reactor, additionally contains different amounts ofdissolved NH₃ and CO₂, and condensation by-products and unreacted urea.The melamine thus obtained is then solidified, for example, by quenchingwith water or with ammonia, by sublimation with subsequent desublimationor by releasing the pressure under specific conditions.

The reactor used is customarily a tank reactor with a central pipe andheating elements arranged outside the central pipe, which provide theheat necessary for the reaction. These heating elements are pipebundles, in which a salt melt circulates, arranged parallel to thecentral pipe. Urea and NH₃ are introduced at the bottom of the reactor,impinge on a distributor plate which is located underneath the centralpipe and react in the free space between the pipe bundles, in whichmelamine is already situated, with decomposition and evolution of gas togive melamine. In WO 99/00374, such a reactor is depicted schematically,the flow direction of the melt also being indicated such that thereaction mixture outside the central pipe flows upwards between the pipebundles and separates there into off-gas and liquid melamine. Theoff-gas is removed at the top of the reactor, one part of the melaminemelt is removed from the reactor via an overflow and the other part ofthe melamine melt flows downwards within the central pipe on account ofgravity.

This previously used type of reactor, however, has the disadvantage thatthe pipe bundles, in particular in the case of relatively high ureathroughputs, corrode relatively rapidly and therefore have to befrequently exchanged.

SUMMARY OF THE INVENTION

Unexpectedly, it has now been found that the corrosion rate of the saltmelt pipes can be significantly lowered if the mixture of urea withmelamine and its decomposition takes place not outside, but inside thecentral pipe. Contrary to the original assumption that the flowdirection of the melamine melt is such as indicated in WO 99/00374, ithas been found that the flow direction of the melamine melt in thearrangement according to the invention is exactly the reverse, the meltin fact flows upward within the central pipe and downward outside thecentral pipe.

The supply of heat necessary for the overall endothermic reaction takesplace by means of the heating pipes arranged outside the central pipeduring the movement of the melt downward, so that in the lower part ofthe reactor an approximately 3-30° C., preferably 5-15° C., highertemperature prevails than in the upper part. The fact that the melaminemelt in the upper part of the reactor, where it is removed via anoverflow, is colder than in the lower part means a further advantagecompared with the arrangement according to WO 99/00374, since themelamine melt in the subsequent sections has to be cooled less, and theequilibrium position of the melt at the lower temperature is shifted inthe direction of the melamine, so that fewer by-products are formed.

The invention accordingly relates to a process for the production ofmelamine by pyrolysis of urea in a high-pressure reactor having avertical central pipe with formation of a melamine melt, which ischaracterized in that

-   -   the melamine melt circulating in the reactor mixes in the lower        region of the reactor with a urea melt introduced into the        reactor from below and optionally introduced NH₃,    -   the reaction mixture formed, consisting essentially of melamine,        NH₃, CO₂ and optionally reaction intermediates, flows upwards        from below in the central pipe,    -   the reaction mixture formed emerges from the central pipe in the        upper part of the central pipe,    -   the separation between melamine and off-gas takes place at the        top of the reactor above the central pipe,    -   a part of the melamine emerging at the top from the central pipe        flows downwards in the annular space between the central pipe        and reactor wall and the remainder is expelled for further        work-up,    -   the off-gases are expelled at the top of the reactor.

DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawing, wherein:

FIG. 1 is a side schematic view of a reaction according to theinvention.

FIG. 2 is a side view of a section of a reactor according to theinvention with a cone-shaped distribution plate and flow guide plates.

FIG. 3 a is a side view of an alternate reactor according to theinvention show introduction of a urea melt.

FIG. 3 b is an end cross-sectional view of the reactor of FIG. 3 a.

FIG. 4 is a cross-sectional view of an alternative reactor according tothe invention showing introduction of urea via annular gaps.

DETAILED DESCRIPTION

To carry out the process according to the invention, urea, whichpreferably arrives as ammonia-saturated urea melt from a urea scrubber,having a temperature of approximately 135-250° C. is introduced into themelamine reactor from below. Together with the urea, NH₃ is optionallyintroduced into the reactor from below. The molar ratio of the NH₃optionally fed to the melamine reactor to the urea fed is approximately0-10 mol, preferably approximately 0-5 mol, more preferablyapproximately 0-2 mol of NH₃/mol of urea. The pressure in the melaminereactor, depending on the chosen temperature range, ranges fromapproximately 50-350 bar, preferably from approximately 80-250 bar.

The temperature in the melamine reactor, depending on the chosenpressure range, ranges from approximately 320-450° C., preferably fromapproximately 320-400° C., more preferably from approximately 330-380°C.

The melamine reactor is a tank reactor having a vertical central pipe.The urea melt introduced into the central pipe from below and theoptionally introduced NH₃ preferably flow against a distribution plateinstalled in the lower part of the central pipe and then further eitherpast the distribution plate or through openings or nozzles which arearranged in a retaining device, for example a retaining plate for theattachment of the distribution plate, on the inlet pipe for urea andNH₃, through the distribution plate in the direction of the centralpipe. The reactants mix in the interior of the central pipe with themelamine melt circulating in the reactor and likewise flowing into thecentral pipe from below.

As a result of the intensive mixing of the cool urea melt with the hot,circulating melamine melt in the central pipe, warming of the reactantsoccurs, and the urea pyrolyses over the height of the reactor to givemelamine and off-gas, mainly containing NH₃ and CO₂. Since the formationof melamine is endothermic, the amount of the melamine circulating inthe reactor must be so large that, as a result of the lowering of thetemperature of the melamine during the mixing of the reactants andduring the pyrolysis of the urea, the danger of solidification of themelamine does not exist.

The desired temperature profile in the reactor can be established bymeans of the amount of urea introduced, the temperature of the salt meltand the direction of circulation of the salt melt in the double-jacketpipes.

In addition, it is possible to attach, at the bottom of the reactor orin the central pipe itself, fittings, distribution plates or flow guideplates or the like, which make possible a comparative moderation of theflow in the re-routing of the melamine melt from the annular space intothe central pipe, a better distribution of the melt flows and thecomparative moderation of the bubbles within the central pipe, and alsoa better separation between melamine melt and off-gas on emergence fromthe central pipe and at the top of the reactor.

In the upper part of the reactor, the separation between off-gas andliquid melamine takes place. The melamine melt can emerge there both atthe upper end of the central pipe and additionally through side openingsin the central pipe into the annular space between the central pipe andreactor inner wall.

A part of the melamine flows downward in this annular space, while theremaining melamine melt is expelled from the reactor via an overflow forfurther work-up. The off-gases are continuously removed at the top ofthe reactor, preferably in the direction of the urea scrubber.Advantageously, in the region of the separation between off-gas andliquid melamine, baffles or gratings are arranged as a calming zone andfor the improvement of the separation action.

In the annular region between the central pipe and the reactor wall areusually situated vertical heating pipes that aid in providing to thereactor the amount of heat necessary for the endothermic reaction. Apart of the melamine melt overflowing from the central pipe movesdownwards in the annular space on account of the higher density andmixes again with introduced urea in the lower central pipe region, whichbrings about an internal circulation in the reactor.

The remaining melamine, which is continuously discharged via an overflowat the top of the reactor, is worked up in any desired manner andsolidified. This can be carried out, for example, by releasing thepressure of the melamine saturated with ammonia at a temperature whichlies barely above its pressure-dependent melting point, bysolidification in a fluidized bed or by quenching with water, withliquid or gaseous ammonia or by sublimation and subsequent desublimationfrom the gas phase.

A further aspect of the invention is a reactor for the production ofmelamine by pyrolysis of urea, comprising a vertical reactor body havinga central pipe, feed lines for urea and optionally NH₃ attached in thelower part of the reactor, drain lines for the melamine formed and forthe off-gases containing NH₃ and CO₂ attached in the upper part of thereactor, heating appliances and measuring and control appliances, inparticular for temperature, pressure, flow quantities and height levelof the melt, characterized in that one or more outlet openings for thesupply of urea melt and optionally NH₃ are arranged within the centralpipe.

In the lower part of the central pipe, a distribution plate for thedistribution of the inflowing urea and of the optionally introduced NH₃is preferably installed. The distribution plate can either be designedas a flat plate, or alternatively, for better distribution of theupwardly flowing urea stream and of the likewise upwardly flowingmelamine stream, have any desired geometrical shapes, such as, forexample, the shape of a pyramid, of a half-dish or preferably the shapeof a cone.

It is particularly advantageous if the gap between the distributionplate (3) and the outlet opening of the inlet pipe for the urea melt,and optionally NH₃, is as small as possible, for example an annular gapof about 3-13 mm cross-section or openings or nozzles which are arrangedin a retaining device, for example a retaining plate for the attachmentof the distribution plate to the inlet pipe for urea and optionally NH₃.The openings or nozzles can have any desired geometrical shape and are,for example, circular, annular or in the shape of an annular gap. Theopenings or nozzles are dimensioned such that the emergence rate at theopenings or nozzles is about 0.2-10 m/sec, preferably about 1-5 m/sec,more preferably about 0.5-1 m/sec, so that the reactants in the melamineare finely divided. With this arrangement, a higher emergence rate forurea, and optionally NH₃, is achieved, which makes possible a better,more intensive and even more homogeneous mixing of the reactants withthe outwardly flowing melamine melt. After the emergence from the inletpipe, the urea stream is preferably diverted in the direction of thecentral pipe, such that it flows in the same flow direction as themelamine.

The flowing-in of the melamine melt circulating in the reactor from theannular space between the reactor wall and central pipe into the mixingzone with the urea melt, and the optionally introduced NH₃, can be madepossible, for example, by side openings in the lower region of thecentral pipe.

One possible embodiment of the reactor having a flat distribution platein the central pipe is shown schematically in FIG. 1. FIG. 2 shows apreferred embodiment of the distribution plate in the form of a cone andthe incorporation of flow guide plates. FIGS. 3 a and 3 b show, in theupper part, the introduction of the urea melt by means of nozzles intothe interior of the central pipe and in the lower part a cross-section,FIG. 4 shows the introduction of urea via annular gaps in thecross-section.

In FIGS. 1 to 4, the following reference numerals are used: (1) melaminereactor, (2) central pipe, (3) distribution plate, (4) heating pipe, (5)annular space, (6) flow guide plates, (7) supplied urea melt, (8) NH₃gas, (9) off-gases, (10) melamine melt for further work-up, (11)fittings, (12) baffle, and (13) nozzle or annular gap.

The reactor comprises a corrosion-resistant material or is lined withcorrosion-resistant material, for example titanium.

It is possible to attach, at the reactor bottom, in the central pipeand/or in the separating zone at the top of the reactor, fittings,distribution plates, flow guide plates or the like, which make possiblea comparative moderation of the flow in the redirection of the melaminemelt from the annular space into the central pipe, a better mixing ofurea and melamine melt, a comparative moderation of the bubble sizewithin the central pipe and on emergence from the central pipe, and abetter separation between melamine melt and off-gas at the top of thereactor.

The vertical heating pipes (4), through which the heat necessary for thereaction is provided, are preferably double-jacket pipes in which a saltmelt circulates. Here, the supply of the salt melt can be carried outeither via the inner pipe cross-section and the discharge via the outerpipe jacket or in the reverse flow direction.

As a result of the mixing and reaction of the reactants within thecentral pipe, their corroding action comes to bear to a much lesserextent. For example, in a melamine reactor according to the presentinvention, such as shown schematically in FIG. 1, the decrease in thepipe wall thickness of those heating pipes (4) which lie next to thecentral pipe (2) is approximately 0.1 mm/year at a capacity of 2.5 t ofmelamine/h. In comparison, the reduction in the pipe wall thickness withthe same high throughputs, but with mixing of the reactants outside thecentral pipe (2), is up to approximately 0.9 mm/year.

The melamine melt circulating in the reactor serves as a heat-transfermedium for the urea melt introduced into the reactor. Here, an overalldecreasing temperature profile is established corresponding to theproceeding urea pyrolysis reaction over the height of the central pipe,i.e. in the vicinity of the melamine overflow from the reactor a lowertemperature prevails than at the reactor bottom. The melamine outlettemperature from the reactor is therefore lower than in most melamineprocesses. It preferably ranges from 330 to 380° C., more preferablyfrom 340 to 370° C. A particular advantage of the reverse flow directionis the low outlet temperature of the melamine melt from the synthesisreactor, which can only thus be run at a lower temperature than in thepreviously known processes. The melamine synthesis reactor acts in theupper part as a precondenser. Thus the precondensed melamine meltseparated from the off-gases arrives even from the start with a lowerby-product content for the next working-up steps.

Moreover, as a result of the clean liquid flow—as opposed to a two-phaseflow with reversed direction of circulation of the melamine melt—alowering of the pressure loss in the annular space between the salt meltpipes, and thus an increase in the amount of circulation in the reactor,is achieved. The heat transfer from the salt melt to the melamine meltis thereby improved.

In addition, the possibility exists with fittings in the central pipe ofinfluencing the flow and the bubble size and distribution of theoutwardly flowing reaction mixture by means of which a furtherimprovement in the substance and heat transfer can be achieved.

1. A reactor for the production of melamine by pyrolysis of urea,comprising: a vertical reactor body having a central pipe, feed linesfor urea melt and optionally NH₃ installed in a lower part of thereactor, drain lines for melamine melt formed and for off-gasescomprising NH₃ and CO₂ in an upper part of the reactor, and heatingappliances and optionally measuring and control appliances, wherein oneor more outlet openings for the supply of urea melt and optionally NH₃are arranged in the lower part of the reactor and within the centralpipe.
 2. A reactor according to claim 1, further comprising adistribution plate in a lower part of the central pipe, above the feedlines for urea and optionally NH₃.
 3. A reactor according to claim 2,wherein the distribution plate is provided with one or more openings inthe direction of the central pipe for the passage of the urea melt andoptionally NH₃.
 4. A reactor according to claim 2, wherein thedistribution plate is in the form of a flat plate, a cone, a pyramid ora half-dish.
 5. A reactor according to claim 1, further comprisingfittings situated in the central pipe for the comparative moderation ofthe flow and the comminution of the gas bubbles and for the improvementof the mixing.
 6. A reactor according to claim 1, further comprising abaffle and, above the baffle, a calming zone situated at the top of thereactor above the central pipe in a separation zone.
 7. A reactoraccording to claim 1, wherein the central pipe has side openings in itslower and/or upper part.